As known in the art, ECG/EEG systems measure the voltages on a skin surface of a patient. These measured voltages are typically less than 1 mV. Of note is a skin electrode interface impedance may vary dramatically from patient to patient. For example, these impedances may vary between 10K ohms and 10M ohms for a typical connection to the patient with standard electrodes. Such high impedances however are problematic with the use of standard electrodes.
More particularly, any electrostatic coupling into a wire that connects the electrode to an amplifier input of a ECG/EEG monitor will result in current flow across the patient impedance, and any artifact signal generated by such an electrostatic coupling is directly proportional to the impedance of the electrode skin interface. To impede the generation of artifact signals, ECG/EEG monitors currently use shielded wires in order to minimize any electrostatic coupling.
In addition to the direct coupling into an individual wire, electrostatic coupling may occur in a common mode coupling to the patient. Specifically, as implemented in the art, ECG/EEG monitors use one electrode as a reference electrode to provide for current flow between the patient and the ECG/EEG monitors. Typically, an active feedback loop is used to force most of the common mode current to flow through the reference electrode and minimize a common mode signal present on the measurement electrodes. The amplitude of the common mode signal is equal to current of the common mode coupling times the reference electrode impedance divided by the loop gain of the active feedback loop. The remaining signal then is cancelled by the matching of the input amplifiers in the input circuitry of the ECG/EEG monitors.
In the case of emergency response where the patient is transported in an ambulance while being closely monitored for ECG, the sources of electrostatic coupling are substantial. For example, many possible sources of statically charged bodies are in the ambulance, and movement of charged bodies when driving down a road is very probable. Consequently, a patient with high contact impedance electrodes will typically have significant artifact present in the ECG when the ambulance is in motion, and the ambulance will have to pull over and stop in order to perform a 12-lead static-free ECG measurement to transmit to the hospital.
Likewise, sources of electrostatic coupling may be substantial for a patient being closely monitored for EEG.